280 TWENTIETH REPORT ON THE STATE CABINET. 



There seemed still something not quite understood in regard to its interior 

 structure ; and it was only as the description was going to press in Vol. iv, 

 following the Strophomenid^, that I resolved to make some further 

 efforts to discover its entire structure ; which resulted in determining 

 the characters of the loop illustrated in figures 5 and 7, page 280. In pla- 

 cing the genus at the end of the volume, and following Terebratula 

 and allied genera, I do not mean to be understood as placing it without 

 reserve among the Terebratultd^ : nevertheless this seems to be its 

 nearest relation, according to our present views of the classification of 

 the Brachiopoda. In thus placing it, I have written as follows (Pal. N. Y., 

 vol. iv) : " We had originally supposed that Tropidoleptus would find 

 its place among the Strophomenidae ; but there have been at all times 

 some important objections to placing it in this relation, while later discove- 

 ries have rendered such a reference unnatural. Notwithstanding the 

 concavo-convex form, area, and large fissure under the beak of the ventral 

 valve, still the punctate structure of the shell and the character of the 

 crura seem to indicate its relations to be with the Terebratulidae ; and 

 I have accordingly placed it in an order following the authentic genera 

 of that family." 



In the ventral valve, the peculiarities lie in the strong crenulate teeth, 

 which are a little separated from the margin of the area (fig. 3). These 

 teeth are quite strong and thickened below, and their crenulate summits 

 are inserted into similarly crenulated teeth-sockets at the base of the 

 strong cardinal process of the opposite valve. In this valve, the divaricator 

 muscular impressions are broadly flabelliform. The occlusor muscular 

 impressions have not been satisfactorily observed. 



The dorsal valve has a narrow area, and a wide and strong cardinal 

 procass which nearly or quite fills the foramen of the opposite valve. 

 T. is process is often simple exteriorly, above the limit of the smooth or 

 striated pseudo-deltidium which covers it near the hinge-line ; but just 

 within the valve it is broadly grooved in the middle, usually with two small 

 deep pits just within the external smooth callosity, and on each side there 

 is a groove and accessory lobe, frequently not conspicuous. The divisions 

 made by the median groove diverge and terminate below in obtuse processes 

 which have some similarity with the bases of crural processes in Orthis, 

 but have more analogy with the Terebratulidae. These processes are some- 

 times clearly broken at their termination, but are often smooth, as if the 

 roughened surface had been cicatrized during the life of the animal. Below 

 these forks of the process there is a narrow median crest or septum which 

 reaches beyond the middle of the valve, and sometimes nearly to the 

 front. From the limbs of the thickened divergent processes there proceed 

 slender crura which, at first bending slightly outwards, send off a short 

 spur into the ventral cavity and are thence directed forwards, and gently 

 curving, join the median crest, to which they are attached, forming a loop 

 of peculiar character. The occlusor muscular impressions have rarely been 

 seen with any degree of distinctness ; but the depressions just at the 



